Two Boston bomb suspects were named as brothers, Dzhokhar A. Tsarnaev, 19, and his brother Tamerlan Tsarnaev, 26, a U.S. national security official said on Friday.

The official said the older brother died in a shootout with police and the younger one was being sought in a house-to-house search for in the Boston suburb of Watertown.

The brothers had been in the United States for several years, the official said. National security and law enforcement officials said they were leaning toward the theory that the bombings were motivated by Islamist extremism.

Boston Suspect's Web Page Venerates Islam, Chechen Independence

Boston bombing suspect Dzhokhar Tsarnaev posted links to Islamic websites and others calling for Chechen independence on what appears to be his page on a Russian language social networking site.

Abusive comments in Russian and English were flooding onto Tsarnaev's page on VK, a Russian-language social media site, on Friday after he was identified as a suspect in the bombing of the Boston marathon.

Police launched a massive manhunt for Tsarnaev, 19, after killing his older brother Tamerlan Tsarnaev in a shootout overnight.

On the site, the younger Tsarnaev identifies himself as a 2011 graduate of Cambridge Rindge and Latin School, a public school in Cambridge, Massachusetts.

It says he went to primary school in Makhachkala, capital of Dagestan, a province in Russia that borders Chechnya, and lists his languages as English, Russian and Chechen.

His "World view" is listed as "Islam" and his "Personal priority" is "career and money".

He has posted links to videos of fighters in the Syrian civil war and to Islamic web pages with titles like "Salamworld, my religion is Islam" and "There is no God but Allah, let that ring out in our hearts".

He also has links to pages calling for independence for Chechnya, a region of Russia that lost its bid for secession after two wars in the 1990s.

The page also reveals a sense of humour, around his identity as a member of a minority from southern Russia's restive Caucasus, which includes Chechnya, Dagestan, Ingushetia and other predominately Muslim regions that have seen two decades of unrest since the fall of the Soviet Union.

He has posted his own joke: "A car goes by with a Chechen, a Dagestani and an Ingush inside. Question: who is driving?"

The answer: the police.

Police Action in Watertown

Authorities warned people in Watertown not to leave their homes and not to answer the door after a night in which a university police officer was killed, a transit police officer was wounded, and the suspects carjacked a vehicle, leading police on a chase.

Police were searching for the man known as Suspect 2 who was photographed wearing a white hat just before the explosions that killed three people and wounded 176. The blasts triggered security scares across the United States and evoked memories of the Sept. 11, 2001 attacks.

The bombing suspects attacked police with explosives and gunfire before the man known as Suspect 1 was shot, apprehended, and taken to a hospital, where he died, officials said.

Officials shut down area transit systems while the manhunt was under way.

"We believe this to be a terrorist," said Boston Police Commissioner Ed Davis of the bombing suspect still at large. "We believe this to be a man who has come here to kill people. We need to get him in custody."

The two unidentified men were wanted for Monday's twin bombings at the Boston Marathon, when two blasts ripped through the crowd near the finish line.

The massive police operation was under way in Watertown after the U.S. Federal Bureau of Investigation (FBI) on Thursday released pictures and video of two suspects in the Boston Marathon bombing.

The FBI had enlisted the public's help in identifying two men wearing backpacks and baseball caps in the crowd minutes before bombs exploded near the finish line.

HOW THE NIGHT BEGAN

About five hours after the FBI released the pictures of the bombing suspects, a police officer was shot and killed on the campus of Massachusetts Institute of Technology, the Middlesex County District Attorney said in a statement.

A short time later, police received reports of a carjacking by two men who kept their victim inside the car for about half an hour, the statement said.

Police pursued that car to Watertown, where explosives were thrown from the car at police and gunfire was exchanged, the statement said.

"During the exchange of the gunfire, we believe that one of the suspects was struck and ultimately taken into custody. A second suspect was able to flee from that car and there is an active search going on at this point in time," Colonel Timothy Alben, superintendent of the Massachusetts State Police, told a news conference.

"What we are looking for right now is a suspect consistent with suspect No. 2, the white-capped individual who was involved in Monday's bombing of the Boston Marathon," Alben said.